FIG. 1 illustrates a fiber optic ring network 10 having a bus 11 connecting a plurality of nodes 12. Each node includes a transmitter 14 and a receiver 16 for injecting data onto and receiving data from the network 10. As shown, all data on the network 10 passes around the ring in one direction from one active node to the next. Since the data is typically received and regenerated at each node 12, if any node 12 is inoperable, the signal on the network 10 must bypass the inoperable node 12 and be propagated to the next node 12. Each node 12 normally receives data from the previous node 12, and if the data is destined for a different node (as determined from encoded destination information within the data) passes that data on to the next node in the ring 10. The data continues to travel around the ring 10 until it arrives at the destination node. Also any node 12 can determine if the previous adjacent node in the ring 10 has gone dead by looking at the signal strength of the received signal. If the signal strength falls below a set threshold, the receiving node knows that the previous node in the ring 10 is not operable and boosts its gain to start receiving data from two nodes back.
Two prior art bypass switches for fiber optic networks are illustrated in FIGS. 2A through 2D and FIGS. 3A through 3D. A bypass relay 22, as shown in FIGS. 2A through 2D, includes a stationary element 24 and a moveable element 26, shown in the on or active position in FIG. 2A. FIG. 2B is a schematic representation of the FIG. 2A position. In the active mode the signal on the optical bus 11 is received by the receiver 16 at the node 12. If the received signal is not destined for that node 12, it is retransmitted by the transmitter 14. Alternatively, the transmitter 14 can inject an original signal onto the bus 11. In FIG. 2A the active node is established by coupling an end 30 of the fiber optic bus 11 to an end 32 that is connected to the receiver 16. Likewise an end 34 of the fiber optic bus 11 is coupled to an end 36 that is connected to the transmitter 14. In FIG. 2C the bypass switch 22 is shown in the off or inactive position. Here the moveable element 26 is shifted with respect to the stationary element 24 so that the end 30 is coupled to the end 34, providing a continuous path for the data on the bus 11 and bypassing the transmitter 14 and the receiver 16. A schematic illustration of the inactive position is shown in FIG. 2D.
FIGS. 3A through 3D illustrate a second bypass switch 40, including a stationary member 42 and a moveable member 44. In FIG. 3A the stationary member 42 and the movable member 44 are aligned so that a signal on the optical bus 11 is coupled to the receiver 16 via opposing end faces 46 and 48. End faces 50 and 52 are also aligned so that the signal from the transmitter 14 is coupled onto the optical bus 11. The schematic representation of the physical arrangement of FIG. 3A is illustrated in FIG. 3B. In FIG. 3C the bypass switch 40 is shown in an off or inactive position where the end face 46 is coupled to the end face 52 for uninterrupted transmissions along the optical bus 11. This arrangement is illustrated schematically in FIG. 3D. Also, in FIG. 3C a fiber optic segment 54 is coupled between the end faces 48 and 50. The fiber optic segment 54 connects the receiver and transmitter directly enabling self-testing of the transmitter 14 and the receiver 16. This embodiment is illustrated schematically in FIG. 3D.
Another prior art embodiment is illustrated in FIG. 4 where the light exiting from an input fiber 58 is focused by a lens 60 onto a beam splitter 62. A portion of the light reflected from the beam splitter 62 is directed toward a lens 64, where it is focused onto a fiber optic segment 66 coupled to the receiver 16. The remainder of the light impinging upon the beam splitter 62 is transmitted therethrough to a beam splitter 68. A portion of the light exits the beam splitter 68 and is focused by a lens 70 onto an outgoing fiber segment 72. To transmit a signal from the node illustrated in FIG. 4 the transmitter 64 is activated to propagate a signal through a fiber optic segment 74 and a lens 76. Light rays from the lens 76 strike the beam splitter 68 and are reflected towards the lens 70 and the outgoing fiber segment 72.
The primary disadvantage of these prior art fiber optic bypass schemes is their complexity, use of multiple parts, and requirement for mechanical movement to move the optical fibers into the desired position for , transmitting or receiving the optical signals or for node-bypassing. The use of moving parts, while suitable in some applications, is not acceptable for fiber optic ring networks intended for adverse environments.